Add comment explaining the new flag.

llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp

static cl::opt<bool> ClCombinePointerLabelsOnStore(
     cl::Hidden, cl::init(false));
 
 static cl::opt<bool> ClDebugNonzeroLabels(
     "dfsan-debug-nonzero-labels",
     cl::desc("Insert calls to __dfsan_nonzero_label on observing a parameter, "
              "load or return with a nonzero label"),
     cl::Hidden);
 
+// Experimental feature that inserts callbacks for certain data events.
+// Currently callbacks are only inserted for stores.
+//
+// If this flag is set to true, the user must provide definitions for the
+// following callback functions:
+//   void __dfsan_store_callback(dfsan_label Label);
 static cl::opt<bool> ClEventCallbacks(
     "dfsan-event-callbacks",
     cl::desc("Insert calls to __dfsan_*_callback functions on data events."),
     cl::Hidden, cl::init(false));
 
 static StringRef GetGlobalTypeString(const GlobalValue &G) {
   // Types of GlobalVariables are always pointer types.
   Type *GType = G.getValueType();